Electric discharge machining has established a solid position as a machining technology for such as dies and molds, and has been extensively used in the field of die and mold machining in the automobile industry, the household electrical appliance industry, the semiconductor industry, and the like.
FIG. 5 is an explanatory diagram of the mechanism of electrical discharge machining. In the drawing, reference numeral 1 denotes an electrode; 2, a workpiece; 3, an arc column; 4, a working liquid; and 5, machining debris produced in electrical discharge machining. Removal machining based on the electric discharge in the workpiece 2 progresses while repeating the cycle of the following steps (a) to (e) (corresponding to steps (a) to (e) in FIG. 5): Namely, these steps are (a) the formation of the arc column 3 due to the generation of electrical discharge, (b) the local fusion and the vaporization of the working liquid 4 due to the thermal energy of electric discharge, (c) the generation of an explosive force of vaporization of the working liquid 4, (d) the scattering of fused portions (machining debris 5), and (e) cooling, solidification, and recovery of insulation in the gap between the electrode 1 and the workpiece 2 due to the working liquid.
This invention concerns wire electrical discharge machining which is used in boring, cutting, and the like in electrical discharge machining which effects removal machining of a workpiece by heating and fusing the workpiece by pulse-like discharges. In particular, there has been a growing demand for higher precision in the wire electrical discharge machining, and high machining accuracy on the order of 1 to 2 μm or thereabouts has come to be required in the machining of high-precision dies and molds used in the semiconductor industry and the like.
FIG. 6 is an explanatory diagram illustrating an example of the machining process of wire electrical discharge machining. In the drawing, reference numeral 1a denotes a wire electrode; 2, the workpiece; 4a, water, i.e., a working liquid; and 6, an initial hole. The part (a) of FIG. 6 shows the state of a first cut which is rough machining, the part (b) of FIG. 6 shows the state of a second cut which is semi-finish machining after rough machining, and the part (c) of FIG. 6 shows the state of a third cut which is final finish machining.
The example of the machining of the first cut in the part (a) of FIG. 6 shows machining in which the wire electrode 1a is passed through the initial hole 6, and the workpiece 2 is bored. In the case of such a first cut, since the surface roughness and accuracy of the machined surfaces of the workpiece are finished in subsequent machining, very strict surface roughness and accuracy are not required so much, and it is important to increase the machining speed, in particular, so as to improve productivity. In wire electrical discharge machining, in order to increase the machining speed, the water 4a is powerfully sprayed to the gap between the wire electrode 1a and the workpiece 2 so as to efficiently discharge the machining debris from the gap. In addition, in order to eliminate the unevenness of the application of the water 4a to the gap and prevent the disconnection of the wire electrode 1a, a method is adopted in which the water 4a stored in an unillustrated working tank and the workpiece 2 is immersed in it.
In the above-described conventional wire electrical discharge machining, the machining after the first cut (the part (a) in FIG. 6), such as the second cut (the part (b) in FIG. 6) and the third cut (the part (c) in FIG. 6), is also effected in the water 4a, i.e., the working fluid.
FIG. 7 shows examples of the voltage and the current waveform in the gap between the wire electrode 1a and the workpiece 2. In the drawing, V denotes a gap voltage; I, a gap current; and t, time. The state at a timing T1 in FIG. 7 is a state in which the voltage is applied across the gap. When a voltage is applied across the gap, a force acts in which the positive polarity and the negative polarity are attracted toward each other, so that the wire electrode 1a having small rigidity is pulled toward the workpiece 2 side by this electrostatic force. This causes the vibration of the wire electrode 1a, so that there has been a problem in that high-accuracy machining is made difficult due to such vibration.
In addition, the state at a timing T2 in FIG. 7 is a state in which the explosive force of vaporization of the working liquid has been generated due to the discharge energy (e.g., the part (c) in FIG. 5), wherein a large force acts on the wire electrode 1a in a direction opposite to that of the workpiece 2 due to the explosive force of vaporization of the working liquid, so that vibrations occur. There has been a problem in that irregularities occur in the shape of the workpiece 2 due to such vibrations, which leads to the deterioration of the accuracy.
In the semiconductor industry and the like, which are the fields of application of wire electrical discharge machining, in the machining of such as a die for IC leadframes, applications are increasing in which extremely high accuracy and very smooth surface roughness are required for workpieces whose form accuracy is 1 μm and whose surface roughness is 1 μm Rmax or less. In such uses, in particular, the above-described problem ascribable to the vibration and the like of the wire electrode has been noticeable.
As a measure for overcoming such problems of wire electrical discharge machining in such a working liquid, a technique concerning aerial wire electrical discharge machining has been disclosed in which wire electrical discharge machining is performed in the atmosphere without a working liquid interposed in the gap between the wire electrode and the workpiece (Adachi, Tokyo University of Agriculture and Technology, et al.: “Attaining High Precision in Second Cuts by Aerial EDM,” Die & Mold Technology, Vol. 14, No. 7, 1999, p. 154, The Nikkan Kogyo Shimbun, Ltd.). However, although it is disclosed that accuracy in the straightness of cut surfaces of workpieces is improved by wire electrical discharge machining in the atmosphere, no disclosure is given of a specific configuration and the like concerning a measure for coping with applications in which higher accuracy is required and applications in which high quality of the workpiece surface is required.